OK, I think I'm starting to understand how this works now. My understanding is:

1. Certain types of files can be accessed directly from the SD card. Such files include videos, music, and photographs.

2. For other types of files, it's necessary to copy them to the appropriate app directory in the internal storage in order for them to be seen. This includes Kindle books. In that respect, it's the same as "Kindle for PC", which can only read books which are in the "My Kindle Content" folder.

Can anyone confirm that this is correct?

From what I understand, Samsung devices when booting scan all storages for the usual media files.
You even see some message "scanning for media files" or something like that. (One of the reasons, why I'm not a Samsung fan. On some of my units it takes forever...)
But: They only look for the common files, such as JPEG or MP4. I guess, basically all the files the tablet can process natively.
This does not work for other file formats, such as Kindle books.

Re. SD cards, I highly recommend the app "App2SD". You can do the very same in the Android settings. But then you have to do it for every single app.
Within App2SD, you see all the apps (plus respective content), that can be moved to SD card.
Unfortunately, on Galaxy Note for example this doesn't work.
Instead of "internal storage" and "SD card", there additionally is some nonsense "USB storage". Instead of moving to SD card, it's moving to the USB storage instead, which basically simply is a partition of the internal storage.
I only had this on some Samsung units and on Sony Tablet S. Strange enough, on Sony Tablet P it's working, not so on Sony Tablet S.

There are some relatively complicated hacks to link from one storage type to another one. Theoretically, everything could be moved from internal storage to SD card that way.